SH-waves at a corrugated interface between a dry sandy half-space and an anisotropic elastic half-space

Summary A problem of reflection and transmission of a plane SH-wave incident at a corrugated interface between a dry sandy half-space and an anisotropic elastic half-space is investigated. Rayleigh's method of approximation is applied to derive the reflection and transmission coefficients for the first and second order approximation of the corrugation. The expressions for reflection and transmission coefficients for the first order approximation of the corrugation are obtained in closed form for a special type of interface, and the energy partition relation is derived. It is found that these coefficients are proportional to the amplitude of corrugation and are functions of elastic properties of the half-spaces and also of the angle of incidence. Numerical examples illustrating the effects of the sandiness, the anisotropy, the corrugation of the interface, the frequency, and the angle of the incidence on the coefficients are presented.     

Charge Transport in Unconventional Superconductor Junctions

We report two remarkable recent topics about zero-bias conductance peak (ZBCP) in normal-metal/unconventional superconductor junctions. We show that the roughness at a surface of superconductors strongly suppresses the ZBCP when the transparency of the interface is sufficiently low. In a numerical simulation at a zero-magnetic field, we confirm the split of the ZBCP owing to the interfacial roughness in realistic band structures of the high-T _c superconductors. We also study the influence of a magnetic field H on the zero bias conductance peak (ZBCP). For p-wave junctions, ZBCP does not split into two by H even for sufficiently low transparent junctions, where ZBCP clearly splits for d-wave. This unique property originates from the fact that for p-wave superconductors, perpendicularly injected quasiparticle form ZES, which contributes most dominantly on the tunneling conductance. We propose that tunneling spectroscopy in the presence of magnetic field, i.e., magnetotunneling, is a promising method to determine the pairing symmetry of unconventional superconductors.     

Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions

An intrinsic gap depression at the superconductor-insulator interface due to the very short value of the coherence length in high-T _c superconductors [HTS] is considered in the framework of a mixed (s+id)-wave pair symmetry for the order parameter ranging from pures to pured-wave. This gap depression acts as the main physical agent causing the relevant reduction ofI _c (T)R _n (T) values with respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with various experimental data is obtained with both pures-wave and pured-wave symmetries of the order parameter, but with amounts of gap depression depending on the pair symmetry adopted. Regardless of the apir symmetry considered, these results prove the importance of the surface order-parameter depression in the correct interpretation of theI _c (T)R _n (T) data in HTS SIS junctions. In the case of a planar YBCO-based junction the use of the de Gennes condition allowed us to tentatively obtain an upper limit for the amount ofd-wave present in the gap of YBCO.     

Diffraction d'ondes SH par un milieu stratifié sphérique

The interactions of elastic SH waves with a given set of m concentric elastic spherical layers immersed in an infinite, homogeneous, isotropic, elastic space are studied. The expansion of the generalized Debye potential which corresponds to the exact solution of this scattering problem is obtained directly from the solution of two simpler problems. From this expansion is extracted the series of elements which corresponds to waves which have interacted with the interface below any layer. A detailed asymptotical study of this series is given. The results of this study are interpreted in terms of geometrical optics.     

The kinked interface crack

The integral equation of the kinked interface crack is solved numerically. The values of K _I, K _II and G for an interface crack with an infinitesimal kink are used to predict the kinking angle for two different material combinations under uniaxial tension.     

Effect of metallization on the power flow angle of SH0 waves in thin piezoelectric plates

The anisotropy of the angle between the phase and group velocity (power flow angle, PFA) as well as the influence of electrical boundary conditions on this angle have been studied for shear-horizontal fundamental (SH₀) acoustic waves in thin piezoelectric plates of lithium niobate (LNO) and potassium niobate (KNO). Both LNO and KNO crystals possess orientations at which the PFA for SH₀ waves reaches large values (17° and 48°, respectively). It is established that metallization of the piezoelectric plate surface can significantly change PFA values, in particular, by 18° and 55° for Y-X + 25° cut plates of LNO and KNO, respectively.     

Symmetry of Superconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>: <Emphasis Type="Italic">s</Emphasis> or <Emphasis Type="Italic">d</Emphasis>

The superconducting hole condensate of YBa₂Cu₃O₇ resides in its BaO layers, and involves s-wave paired holes. This picture differs from the currently widespread opinion that the superconductivity at the surface and in the bulk is d-like, and resides in the CuO₂ planes: it is the main reason why high-temperature superconductivity has been unsolved.     

Flux Phase and Its Competition with Superconductivity in the t–J Model

The phase diagram of the t–J model is investigated within the X-operator formalism using the Baym–Kadanoff theory and a 1/N expansion. In this way, no auxiliary fields are introduced. For finite Coulomb repulsion, the system shows a strong competition between d-wave superconductivity and d-wave flux phase, leading to a strong suppression of superconductivity in the flux state. The underdoped region is characterized by flux order in the normal state, with a d-wave gap in the excitation spectrum, and by coexistence of both superconductivity and flux phases below the superconducting transition temperature T _c. The balance between the two phases is determined by the short-range Coulomb repulsion, while the long-range part of Coulomb interaction prevents phase separation and leads to incommensurate charge-density-wave state far away from the superconducting region.     

Collective modes and a theory of response ofd-wave superconductors

We present a theory of the response ofd-wave superconductors to weak applied fields, by taking account of the Coulomb interaction and all the collective degrees of freedom as well as crystal symmetry. We choose two representative phases: the d phase, which has point nodes in the energy gap, and theY _2–1 phase, which has line as well as point nodes. The former is a self-consistent solution for cubic as well as spherical symmetries and the latter is one for spherical, cubic, and hexagonal symmetries. We obtain obviously gauge-invariant expressions for the order-parameter fluctuations and the currents, having forms common not only to thed-wave states, but also to thep-wave states studied earlier. We also investigate the collective excitations; in the long-wavelength limit for spherically symmetric systems, there are, on the frequency-temperature plane, seven branches for eachd-wave phase considered, in addition to the common plasma mode and orbital Goldstone modes resulting from the spontaneous breakdown of the rotational invariance. In theY _2–1-phase two eigenmodes are found to become gapless at a finite temperature, below which they are purely imaginary. This implies instability of the phase. The effect of crystal anisotropy on the collective spectra is also studied.A preliminary report on the present work was published inJpn. J. Appl. Phys. Suppl. 26-3, 167 (1987).     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Non-linear Localized Lattice Mode Coupling Mechanism and the Pseudogap in High-temperature Superconducting Cuprates

The pseudo-gap phenomena in high-temperature cuprate superconductors is an outstanding puzzle with no consensus yet on its physical origin. A previous suggestion on the role of non-linear local lattice instability modes on the microscopic pairing mechanism in high temperature cuprate superconductors is re-examined to investigate whether unusual lattice mechanisms could cause a pseudo-gap. By assuming an electron predominantly interacting with a non-linear Q ₂ mode of the oxygen clusters in the CuO₂ planes, we show that the interaction has explicit d-wave symmetry and leads to an indirect coupling of d-wave symmetry between electrons. We show that the polaron formation by the non-linear mode can cause a pseudo-gap of d-wave symmetry before the onset of coherence in the superconducting pairing. We suggest a simple phenomenological explanation of the pseudo-gap crossover temperature and the Fermi arcs. The discussion may be relevant for the pseudo-gap in non-superconducting giant magnetoresistive manganites.     

The influence off-wave pairing fluctuations on sound propagation in superfluid3He-A

The theory of sound propagation in pair-correlated Fermi liquids developed previously by Wölfle, with additionalf-wave pairing fluctuations, is applied to the ABM state. Expressions for the anisotropic sound absorption and velocity at arbitrary temperature and frequency in the collisionless limit are derived. Thef-wave pairing fluctuations give a large effect on the normal-flapping mode frequency at low temperatures. The corresponding shift in the sound attenuation peak of this collective mode provides a sensitive probe of thef-wave pair coupling constantg ₃. There is also a pronounced effect on the super-flapping mode attenuation peak, which becomes well defined whenf-wave pairing fluctuations are considered.     

Skew plane shock wave at the interface between two polytropic gases. A heavy-light gas system

In [Inzh.-Fiz. Zh., 72, No. 2, 208–215, 216–225 (1998)] the problem of break decay that occurs when a plane skew shock wave (SSW) arrives at the interface between two polytropic gases of different density (the polytrope indices of both gases remain constant in the entire region of interaction) from the side of the less dense of them is considered in detail. The change in the stationary shock-wave configuration with an increase in the angle of incidence from 0 to π/2 is tracked. In the present work, consideration is given to the development of a stationary shock-wave configuration in the case where the SSW emerges at the interface from the side of the more dense gas. Problems associated with boundary layers and mixing of contacting gases were not considered. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 4, pp. 802–806, July–August, 2000.     

Anisotropic superconductivity in the Emery model

We have investigated anisotropic superconductivity originating from intersite pairing between holes in nearest and next nearest neighboring sites in the Emery model. Strong local Coulomb correlations among holes in copper orbitals have been taken into account within the Hubbard I approximation scheme. The superconducting transition temperature has been evaluated as a function of the hole concentration. It has been shown that with the onset of superconductivity, pairing among oxygen-like quasiparticles in the mixeds-wave+d-wave channel plays the dominating role, being replaced by pairing in the extendeds-wave channel for higher concentration of holes. Superconducting correlations are mostly effective for a rather narrow range of the model parameter values, close to values derived from band structure calculations. Therefore, the coupling betweens-wave andd-wave channels seems to be a general feature of superconductivity in CuO₂ planes.     

Interface roughness influence on exchange bias effect in La<Subscript>2/3</Subscript>Ca<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>1/3</Subscript>Ca<Subscript>2/3</Subscript>MnO<Subscript>3</Subscript> bilayers

A Monte Carlo simulation study of La_2/3Ca_1/3MnO₃/La_1/3Ca_2/3MnO₃ bilayers exchange bias (EB) properties by using a classical Heisenberg model and Monte Carlo method with Metropolis algorithm is addressed. Samples were built atom-by-atom in order to resemble the real roughness. In this model, several contributions included nearest neighbors exchange interactions; two different interface couplings, magnetocrystalline anisotropy and Zeeman term, were considered. Here, an influence of the relaxation steps on the interface roughness is present. Our study focuses on the influence of interface roughness on hysteresis loops, particularly EB field (H _ex) and coercive force (H _c). Results reveal that H _ex and H _c decrease as the interface roughness increases.     

Repulsion and Attraction in High <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductors

The influence of repulsion and attraction in high-T _c superconductors to the gap functions is studied. A systematic method is proposed to compute the gap functions using the irreducible representations of the point group. It is found that a pure s-wave superconductivity exists only at very low temperatures, and attractive potentials on the near shells significantly expand the gap functions and increase significantly the critical temperature of superconductivity. A strong on-site repulsion drives the A _1g gap into a B _1g gap. It is expected that superconductivity with the A _1g symmetry reaches a high critical temperature due to the cooperation of the on-site and the next-nearest neighbor attractions.     

Cuprates and Ruthenates: Similarities and Differences

There is growing evidence that the p-wave superconductivity of Sr₂RuO₄ occurs primarily in the planar -band. Thus the minimum model for both cuprates and ruthenates is a single active band with onsite Coulomb interactions. Recent renormalization group analysis shows that such a model can show singlet d-wave or triplet p-wave pairing. The energy of the van Hove singularity in the band and the shape of the Fermi surface are the decisive factors at weak to moderate interaction strengths.     

Analysis of the Pairing Interaction in the Cuprates

Thermal difference reflectance (TDR) spectra taken on a large number of superconducting cuprates has enabled us to determine the energy dependence and strength of the pairing interaction in each. All show strong contributions from the phonons and a smaller, but significant contribution from an electronic transition near 1.7 eV. Recent improvements in the signal-to-noise ratio have revealed that the electronic excitation is accompanied in all cases by a weaker companion about 0.5 eV lower in energy. No other contributions are found. We identify these transitions as the d _z2 to d _x2–y2 excitations of the Cu ion in the 3d ⁹ and 3d ⁸ states, respectively, and have calculated the coupling strength mediated by each. The calculated values agree in order of magnitude with the observed strengths. We find that, in addition to the direct electronic coupling that corresponds to an electron–phonon-like term, the overlap between the oxygen and the copper orbitals leads to an exchange term. The direct term couples only to an s-wave gap, as for phonons, while the exchange term couples s-wave to d-wave, and vice versa. We discuss the consequences of this result.     

Relation Between Vortex Pinning Energy and Anderson's Theorem

We discuss the elementary vortex pinning in type-II superconductors in connection with the Anderson's theorem for nonmagnetic impurities. We address the following two issues. One is an enhancement of the vortex pinning energy in the unconventional superconductors. This enhancement comes from the pair-breaking effect of a nonmagnetic defect as the pinning center far away from the vortex core (i.e., the pair-breaking effect due to the non-applicability of the Anderson's theorem in the unconventional superconductors). The other is an effect of the chirality on the vortex pinning energy in a chiral p-wave superconductor. The vortex pinning energy depends on the chirality. This is related to the cancellation of the angular momentum between the vorticity and chirality in a chiral p-wave vortex core, resulting in local applicability of the Anderson's theorem (or local recovery of the Anderson's theorem) inside the vortex core.     

Development of an Omnidirectional Shear Horizontal Wave Transducer for Structural Health Monitoring

In this work, an omnidirectional SH₀ mode transducer is proposed. It is based on a simple arrangement of common piezoelectric ceramics attached to a steel disc, and is capable of generating a relatively high ratio of SH₀ to other modes omnidirectionally. A parametric study of the components which constitute the transducer was performed through finite element modelling in order to characterize and investigate the influence of geometry on its frequency–response. The validation of modelling results was performed through the construction of two transducers with different configurations. Analysis of the relationship between the intensity of each mode generated in a plate and the characteristics of the SH₀ mode emission allowed the identification of frequencies that presented the most desirable response.